1. Field of the Invention
The present invention relates to a backlight circuit, and more particularly to a liquid crystal display (LCD) backlight circuit with multiple lamps.
2. Description of the Related Art
LCD panels are used in various applications ranging from portable electronic devices to fixed location units, such as video cameras, automobile navigation systems, laptop PCs and industrial machines. The LCD panel itself cannot emit light but must be back lighted by a light source. The most commonly used backlight source is a cold-cathode fluorescent lamp (CCFL). Usually, a high alternating current (AC) signal is required to ignite and run the CCFL. To generate such a high AC signal from a direct current (DC) power source, e.g., a rechargeable battery, a DC/AC inverter is designed.
However, in recent years, there has been increasing interest in large size LCD displays, as required in LCD TV sets and computer monitors, which require multiple CCFLs to provide necessary illumination. Usually, the DC/AC inverter drives multiple CCFLs coupled in parallel and the CCFLs may also be configured in other ways. One parallel configuration is the direct parallel connection of the CCFLs. This configuration has the well-known problem that CCFL currents may not be balanced owing to the lamp voltage variation and the constant voltage load characteristic of the CCFLs. The imbalance of CCFL currents causes a reduced lifetime of the CCFL and non-uniformity of brightness.
Another parallel configuration is to make the parallel connection at the transformer primary side, as shown in FIG. 1, which illustrates a schematic diagram of a prior art circuit 100 for driving a plurality of CCFLs 140A to 140N. The circuit 100 is composed of a DC power source 110, an inverter circuit 120, a plurality of transformers 130A to 130N, a protection circuit 150 and a controller 160. The inverter circuit 120 is connected to a parallel connection of the primary windings of the plurality of transformers 130A to 130N. The inverter circuit 120 and the plurality of transformers 130A to 130N form an inverter topology, which is well known in the art. The inverter topology converts a DC input voltage VIN from the DC power source 110, e.g., a battery, to a desired AC output voltage VOUT. Those skilled in the art will recognize that inverter topology may be a Royer, a full bridge, a half bridge, a push-pull, and a class D. The AC output voltage VOUT is eventually delivered to the plurality of CCFLs 140A to 140N, which are respectively connected to the secondary windings of the plurality of transformers 130A to 130N.
Furthermore, by sensing the lamp currents IS1 to ISN, the protection circuit 150 may detect a short-circuit condition and then produce a current feedback signal ISEN. By sensing the high side voltages HV1 to HVN of the CCFLs, the protection circuit 150 may detect an open or broken lamp condition in which the CCFL is not connected to the inverter topology, fails to ignite or is broken, and then produce a voltage feedback signal VSEN. The current and voltage feedback signals ISEN and VSEN are then sent to the controller 160 that responses to these feedback signals and takes corresponding actions to prevent damages.
Though the parallel connection at the transformer primary side as illustrated in FIG. 1 may minimize the effect of the lamp voltage variation, which in turn improves the current balance, some drawbacks may impact the performance/cost of the configuration shown in FIG. 1. One drawback lies in that due to the tremendous number of transformers 130A to 130N, the circuit 100 will have an increased cost compared with the configuration of direct parallel connection of the CCFLs. Additionally, elements in the protection circuit 150 for sensing lamp voltages are connected to the high voltage sides HV1 to HVN, which typically have a voltage of more than 1,000 volts. The elements capable of enduring such a high voltage are usually expensive and consequently the overall cost is increased. Furthermore, when connecting the elements to the high voltage sides HV1 to HVN, operators require extra attention to prevent any arcing or hazard. Another drawback lies in that the protection circuit 150 as represented in FIG. 1 is complicated, and the complexity of the protection circuit 150 will become problematic as the number of lamps increases.
FIG. 2A illustrates a schematic diagram of another prior art driving circuit 200A, which is disclosed in U.S. Pat. No. 6,781,325 B2 and can improve the current balance compared with the circuit 100 in FIG. 1. By introducing a plurality of common-mode chokes 250A to 250(N−1), the driving circuit 200A may achieve lamp current balance effectively. Similarly, to prevent potential damages, a protection circuit 260 is included for sensing a short-circuit, open lamp or broken lamp condition. In FIG. 2A, the common-mode chokes 250A to 250(N−1) are respectively connected to the high voltage sides HV1 to HVN of the CCFLs and therefore these common-mode chokes may have a high cost and require extra attention in applications. To reduce the cost and exclude safety concerns, a circuit 200B is configured as represented in FIG. 2B, where the common-mode chokes 250A to 250(N−1) are respectively connected to the low voltage sides LV1 to LVN of the CCFLs.
Though the circuits in FIGS. 2A and 2B may provide a solution to lamp current balance, they fail to overcome the drawbacks relative to circuit protection. Additionally, those skilled in the art will recognize that with the configuration of the multiple transformers in FIG. 1, the currents flowing through the CCFLs may be readily sensed to adjust the brightness of the CCFLs. However, with the one transformer configuration, it is required to specially design a current sense circuit. Furthermore, if the number of the transformers in FIGS. 2A and 2B may be further reduced, significant cost savings will be achieved.